Winning six straight games in a single elimination tournament is infinitely more difficult than winning one game at the end of the season. I mean it's not even close.
How many 7-4 (equivalent to 21-11 in bball) teams have you seen play for a national championship in football?
How many football teams have you seen fail to win their conference and/or their conference championship game, yet still win the national title?
How many lose to 7 unranked teams yet still contended for a title?
How many lose to two different conference teams twice, and another team once but still get a chance to play them AGAIN for the national title?
How often will a team that's not from the south and/or close to a coast get to play for the national title 30 miles from their home?
I think winning both titles is EXTREMELY difficult, and you have to be VERY lucky. However, you have to be both good and lucky ALL YEAR in football. You don't get the luxury of taking a third of the year off only to get hot at the end.
http://en.wikipedia.org/wiki/1987%E2%80%9388_Kansas_Jayhawks_men's_basketball_team